Solid state lighting elements such as light emitting diodes (LEDs) are increasingly being used in a plethora of products including lighting applications due to their low energy consumption and long lifetime. Such applications can be found in many technical areas, such as automotive applications, domestic lighting, commercial applications such as advertising, strip lighting in suspended ceilings and so on. Many more examples will be immediately apparent.
In several applications, the light of the solid state element is coupled into a light guide, where it is manipulated to create a particularly shaped light output.
For instance, U.S. Pat. No. 7,374,323 discloses a display device including a light guide connected to a surface comprising indicia formed thereon and a light-reflecting portion in the light guide disposed at least partially over one of the indicia to illuminate at least one of the indicia, which is used to illuminate indicia in an instrument panel of, for example, a vehicle.
EP 0 006 361 A1 discloses a dial board including light guiding plates that guide the light towards a reflective surface for focussing the light in the vicinity of an opening of the dial board through which spindle of a transparent pointer passes, thereby illuminating the transparent pointer.
JP 2010-134151 discloses light emitting display device in which a planar light guide receives light from LEDs mounted at a side of the light guide. The surface of the light guide comprises a plurality of shapes, which shapes are replicated inside the light guide by respective light emitting layers that extend deeper into the light guide as they are arranged further away from the LEDs. The light emitting layers extract light from the planar light guide and illuminate the shapes on its surface.
A particularly interesting application domain is diffuse lighting panels that produce a substantially homogenous light output over their luminous surface, and more particularly diffuse lighting panels that are transparent in the absence of light injected into the panels, such as the Acrylite® Endlighten sheet marketed by Evonik Industries AG (Essen, Germany), which is embedded with colourless light diffusing particles that scatter light entering an edge of the sheet such that a brightly and uniformly illuminated luminous surface is obtained.
It may be interesting from a functional or aesthetic point of view to create patterns in such a substantially homogeneously lit lighting product. However, the aforementioned prior art solutions all utilize some form of light extraction from a light guide to illuminate a body on the surface of the light guide, which is rather involved and detracts from the aesthetics of the light guide.